Technology from Europe:
United Kingdom

The motor/VGS combination comes of age

The introduction of rotary steerable systems (RSSs) has been heralded – especially by the major directional companies – as the beginning of a new directional drilling age, and in many ways it is. Keeping the assembly turning cleans the hole more efficiently, reduces the chance of sticking and, ultimately, allows drilling further than sliding with a downhole motor. These new tools allow the driller to make subtle corrections as he builds and drops angle, turning left and then right as he follows the geologist's requirements. The only real glitch is getting enough power to the bit from the surface to keep a reasonable ROP. Sometimes, running a motor is preferable.

As ever, engineering is never as clear-cut as one might want. Usually, new tools come with compromises, and this is also true of rotary steerable systems. When working at peak performance in ideal formations for building angle, where excessive vibration, wear and temperature are not a problem, the RSS can perform admirably. With motors still seen as more reliable than RSSs, and some deepwater rigs costing $200,000 per day and more to operate, pulling out of hole from 15,000 ft because a drive shaft has snapped, a pad has fallen off or some other problem becomes an important decision. With RSS prices ranging from $10,000 to $25,000 per day and more, drilling engineers continue to look for cost-effective alternatives.

Motors have improved over the years, but they still have one big problem: You have to stop, orient and then slide to change direction. At lower angles and depths, this is usually not a problem, but going deeper and further afield can be difficult.

At relatively low cost compared to RSSs, motors give power at the bit; but the orient-and-slide problem remains, which is why companies are increasingly looking at combining motors with a variable gauge stabilizer (VGS) for simpler, less tortuous hole sections. Three out of four motor-correction runs are for inclination, rather than azimuth, and it's the same when running rotary steerable systems.

Placing a hydraulically actuated VGS tool above the motor allows building, holding and dropping angle without stopping, orienting and having to slide. This is particularly true when running new, second-generation VGS tools like the hydrastab from Aberdeen-based Toolbox Drilling Solutions Ltd.

This relatively new VGS tool is designed to operate above the motor or in a rotary BHA as a nearbit or first string (control) stabilizer. It is short and the blades are close to the bottom of the tool, which enhances performance when placed above the motor (see picture). It has an “over the hole,” changeable choke assembly that allows the directional man to adjust his surface indication if/when hole conditions dictate. The tool also has a no-fade, no-wash, choke assembly that is designed to maximize the life of the surface signal throughout the run. It takes less than a minute to change gauge, and has an easy-to-read surface indication. This means the directional man is in full control at all times.

This VGS tool comes in various hole sizes, from 5-1/2 -in. to 12-1/4 -in. It offers a 1-1/4 -in. gauge change (12-1/4 -in. hole sections) and the choice of two or three gauge positions, e.g., 11 to 11-1/2 to 12-1/4 -in. Today's longer, more powerful motors need this increased gauge change, if the VGS tool is to be optimally effective. The directional driller wants the ability to build in one position, hold in the second and drop in the third. Using the VGS tool above the motor ensures that the assembly is rotating most of the time (i.e., the motor is only oriented when an azimuth correction is required).

Back in the 1980s, the question asked was: “When should we run the expensive motor, and when should we run a rotary assembly using the VGS as the first string (control) stabilizer?” At that time, VGS tools were mechanical (weight-set), and no one thought that placing it above a motor would reduce slide and orient time. The introduction of simple-to-use hydraulic tools – which do not impede the motor's performance – changed that and showed how inclination corrections could be made simply and effectively without stopping, orienting and having to slide. The question now being asked is, “Where should we run an RSS, and when should we revert to the tried and tested motor/VGS combination?”

Operators need to look carefully at what cost-effective really means. A North Sea operator recently carried out an in-house study on 29 rotary steerable runs made over the past three years. Although by no means a scientific analysis, the conclusion was that only six (21%) could conclusively be shown to have saved the operator money.

Over the same timeframe (2002 – 2003), an estimated 20 RSSs have been Lost in Hole in the North Sea sector. At a cost of about $750,000 per system, that equates to $15 million. There is no question that RSSs are a major boost to directional drilling and, when used to best effect, will drill faster, better wells. With a world-wide PDM inventory of about $750 million, it appears certain that motors will continue to be used in most directional wells. Adding a hydraulically actuated VGS tool essentially yields a 2D rotary system with 3D capability.

The hydraulic variable gauge stabilizer (left); internal cartridge mechanism (right).

Operator experience. ChevronTexaco was the first UK-based operator to run the new VGS above a motor. It has been running these VGS tools for the past three years. In conjunction with Baker Hughes, a new 11-in. tool version has been successfully run on the last four Britannia wells.

A two-positional 11-1/2 -in. tool was run above an SLB motor on the last RWE Dieksand well in Germany. The well was drilled from 11,365-ft. at an angle of 83° to a casing depth of 21,450 ft and an angle of 81°.

Shell Expro runs the tools regularly in the Southern North Sea and offshore/ onshore Holland with NAM. The VGS tools are primarily run above motors to enhance performance, and reduce orientation and slide time.

While not the answer to every problem, this new VGS is one of several second-generation tools that provides a BHA for drilling a section cost-effectively, at one-quarter the RSS day rate, and gives power where it's needed – at the bit.